The total healing is based on your Warframe's maximum health, including mods. For example, with 300 maximum health, you will regenerate 36 health. With 740 health using Vitality, you will regenerate 89 health.
Since the full heal takes four seconds to complete, this mod regenerates 1/4 of the listed percentage per second.
As the health restored is percentage-based, this mod will have a more noticeable effect on Warframes with higher health pools such as Inaros.